This invention relates to a device for the rotational speed-dependent closure limitation of an arbitrarily adjustable carburettor main valve, biased in the closing direction, having a positionally variable throttle stop which, above a rotational speed switching threshold situated above the idling rotational speed, permits a substantially complete closing of the main throttle and thereby interruption to the fuel and air feed and which, below the rotational speed switching threshold, causes an idling minimum opening position of the main throttle, and comprising an electromagnetic switching element governed when the speed falls below the rotational speed switching threshold.
In one known device of this type--carburettor hand book Illgen, fourth edition, 1957, pp 55/56--the electromagnetic switching element is a control magnet, which is energized below the rotational speed switching threshold in order to press the main throttle of a fixed air funnel carburettor into the idling minimum open position via a pivotally journalled crank lever pulled up by the control magnet. Above the rotational speed switching threshold, the control magnet is de-energized, so that the main throttle may be completely closed by means of a closure spring in the thrust phases, that is when the accelerator pedal is released. As a result, the outlet opening of the idling system, which is swept over by the main throttle, is also fully closed. When the rotational speed decreases in the thrust phases and finally falls below the rotational speed switching threshold, the control magnet assures a slight opening of the main throttle, causing the idling system to be brought into operation and the engine not to stop unintentionally. At each renewed rise above the rotational speed switching threshold, the main throttle can once again fully close, which corresponds to a certain control of the idling rotational speed. This known device serves for saving fuel (in the thrust phases) and for regulating the idling rotational speed.
In the known device, in the thrust phases, simultaneously with the fuel feed the air feed is also interrupted, by the main throttle being fully closed. The reason for this interruption of the air feed also is not stated in this prior publication. Within the scope of the present invention it has, however, been established that this reduction, taking place in thrust operation, of the air feed also except for small leakages renders possible a jerk-free decelerating passage through the speed switching threshold. When the main throttle is opened from the thrust position into the idling position, the idling system is swept through even at intermediate positions and caused to deliver. As a result, ignitable mixture enters the engine intake pipe and cylinders even before the idling operation injection is reached. The recommencing first combustion in the cylinders takes place with a reduced residual gas component and is especially intensive. Due to the reduced injection, in spite of the intensive combustion, a rise in the average pressures above those in idling operation is avoided, with the result that onset of combustion takes place without a detectable jerk of the vehicle. With a conventional thrust shutting-down using only the fuel feed, the intensive first combustions in the individual cylinders lead to jerking of the vehicle.
The known device has the series disadvantage that the control magnet must overcome the force of the main throttle closure spring below the rotational speed switching threshold. As a result, relatively large electrical control outputs and a powerful control magnet are required, which latter must often remain energized for long periods during idling and is thus expensive and susceptible to failure. The adjustment speed cannot be varied.